The present invention relates to a plant male sterilant which comprises as an active ingredient an effective amount of a threonine derivative having the formula (I): ##STR2## wherein R.sup.1 is a C.sub.1 to C.sub.2 group; R.sup.2 is a hydrogen atom, a C.sub.1 to C.sub.3 alkyl group or a group having the formula: COR.sup.4 in which R.sup.4 is a C.sub.1 to C.sub.4 alkyl group, a C.sub.1 to C.sub.3 alkoxy group, a phenyl group, an amino group or a C.sub.1 to C.sub.3 alkylamino group; R.sup.3 is a hydroxy group, a C.sub.1 to C.sub.4 alkoxy group or a group having the formula: NHR.sup.5 in which R.sup.5 is a hydrogen atom, a hydroxy group, a C.sub.1 to C.sub.3 alkyl group, an amino group or a C.sub.1 to C.sub.3 alkylamino group and an inert carrier or diluent.
In recent years, the production of hybrid seeds has attracted attention.
It is known that the first filial generation plant has many outstanding characters such as an increased yield compared with its parent variety, owing to its vigorous growth. In order to obtain hybrid seeds it is necessary to prevent self-pollination of a female parent and stamens of the female parent have to be removed for that purpose.
Hitherto, there have been made a lot of efforts for the operation of removing stamens, i.e. castration, and also, since grains having a high rate of self-pollination, e.g. rice, wheat, and the like, have both stamens and pistils in small spikelet, it has been almost impossible to produce a large quantity of the hybrid seeds manually. There are another methods such as use of a cytoplasmic male sterility, but this method has problems such as it takes a long time for its breeding. Therefore, in recent years, it has been desired to develop simple and sure methods to induce male sterility in plant without losing pollination ability of the female parent.
Although O-methylthreonine is described in Winitz et al, J. Am. Chem. Soc. 78, 2423 (1956) and some derivatives thereof are also known, the male sterility inducing activity of O-methylthreonine and its derivatives has not been known at all.
As the result of the continuous effort of the present inventors, now it has been found that a compound having the formula (I) can induce the male sterility in a plant very simply and efficiently by treating the plant with the compound. Consequently the present invention is accomplished.